I have a small property in Gippsland that is mostly covered by Messmate, Apple box and Mountain Grey gum. I have tried to set aside a small section for growing berries, but soil toxicity from gums is most definitely a big issue to overcome as well as acidity and very low nutrient profiles. The latter issues can be easily dealt with, but allelopathy seems to be a more persistant problem.

For this reason I have decided to do some of my own experimentation to better understand eucalyptus allelopathy. I have read Peter Andrews interpretation of allelopathy, and have wondered if it is true that these toxins do leach through the soil.

I had assumed like him that the allelopathic effect from Gum trees was a water-soluble tannin that leaches from the fresh gum leaves. For this reason my first and only experiment to date involved producing tannin water from gum leaves. At first I had thought I had proven that the tannin water was toxic to my experimental seedlings (chicory), however as the experiment progressed it became apparent that the effect I was seeing was not the allelopathic effect I had been seeing in my heavy clay soils. The effect in my experiment was far too weak, and on testing the PH of the tannin water I confirmed that the only effect I was seeing was that of applying an acidic water to potting mix.

I have come across Eucalyptus experiments using a similar method to myself, and which have reported a growth reduction in experimental seedlings, but I am convinced by my work that they too were only seeing the effect of lactic acid on their plants and not true Eucalyptus allelopathy.

On the other hand, I have found some research in which Eucalpytus oil was trialed as a herbicide, and this experiment reports the effects I was looking for in my own experimentation and field observations - discoloration of plant pigments, and very low vigour:

I am not interested in restarting a debate about whether eucalyptus are evil or bad for our landscape. I understand there are some strong feelings about this. For me, Eucalyptus are what I live with, and while I am not afraid to manage them, use them as firewood and take a few out where needed, they are essentially part of my landscape and environment. They, the Koalas, and birds that rely on them have more right to the land than I do. For me, the question is what can I do to improve the soil despite the presence of Eucalyptus.

I thought it worthwhile sharing my preliminary findings with this forum, as allelopathy is such a prominent part of Peter Andrew's writings. However, I think I can be controversial in saying that Eucalyptus allelopathic chemicals essentially only effect the soil they land on and do not usually leach into streams and wetlands. I say this because I believe the toxic effect is only in the oils and not the tannin water which is mostly harmless. Saponins may emulsify these oils and make them more mobile, but it is unlikely that this could carry the toxic effect very far, as once emulsified they will be prone to breakdown quicker, and their concentration in the soils will be too low to be significant.

I believe my humble experiment would suggest we should not over-estimate the allelopathic effect of eucalyptus beyond the area in which they grow. It is certainly fine to grow Eucalpytus where they are suitable and useful without the fear that they will 'pollute' our water down-stream.

I am open to respectful discussion on this before I proceed to new experiments, but based on my belief now that the allelopathy is oil based, I am now much more confident that I can repair some of the affected soils on my property by trialing methods that speed up the break down process and reduce on-going exposure to these oils.

I keep coming back to the finding that the pots with eucalyptus oils and resin acids had a very strong mold growth.

I have also confirmed now that the 'dead' zone around messmates is due to acidity. During a recent rain event I collected some of the water running down the bark and measured its PH at 3.2!

Acidity itself is not necessarily toxic, as organic acids breaks down rocks releasing minerals, but in depleted clays, this acidity lingers and creates dead zones around the trees.

However all the compounds that Eucalyptus produce (including acids) have a tendency to encourage certain fungi to the detriment of other fungi and bacteria.

So this brings me to a range of hypothesizes:

Eucalyptus toxins are too short lived to directly affect other plants, but they are very tasty to certain fungi.These 'toxins' have anti-bacterial properties that help fungi dominate the recycling process. This is a problem for some plants that might rely on bacteria to harness nutrients, ie grasses.

The fungi that Eucalyptus are saprophytic to are very parochial, there are only a few types of plants that can make us of them - some of them being Acacias (particularly Varnish Wattle).

The success of Varnish wattle in the messmate forest is probably because it shares a co-dependency with Messmates through the fungal network - ie the Wattles provide nitrogen and the messmates provide supplementary sugars.

The varnish wattle relationship I have found can be exploited. While varnish wattle has some palatability for goats, another tree legume can also fill the same role - tagasaste!

Tagasaste grows very well under messmates, no doubt getting sugars from them - the implications of this are huge. If this can be generalized to other gums I have found a way of making Eucalyptus shelter belts more productive!

please note that my soils are depleted, and the general recommendation for growing tagasaste is that they need fertilizer inputs! - under messmates they need almost none except a little to get them started!

Now back to allelopathy:

On my property many introduced plants succumb to a form of die-back in summer - the older leaves yellow first, sometimes mottling in the process. The following plants show these signs: avocado, macadamia, tomato, raspberry, walnut, grapes and kiwi-berry (which is now dead!).On the grape vine, the fruit buds also developed fungal disease and withered despite a relatively dry summer.As well as yellowing of the leaves, deciduous plants are likely to hibernate earlier and awaken in Spring much later.

So my hypothesis is that not only have the messmate managed to monopolise the fungal network for the best return of recycled nutrients (the only nutrients my property has!), but the fungi are parasitic on many other plants. The messmates have managed to tame a hostile fungi for their own use.

So in many ways I am questioning now whether Eucalyptus are actually allelopathic in the chemical sense, as apart from some extreme acidity (which is only toxic under certain soil/rock conditions) I don't believe the chemicals that the Eucalyptus produce are particularly harmful to other plants. They are more a morsel for their highly aggressive fungal pets!